spi: pxa2xx: Move chip select control bits into lpss_config structure

Some Intel LPSS SPI controllers, like the one in Braswell has these bits in
a different location so move these bits to be part of the LPSS
configuration.

Since not all LPSS SPI controllers support multiple native chip selects we
refactor selecting chip select to its own function and check
control->cs_sel_mask before switching to another chip select.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>